Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of materials coated upon a substrate. However, as is well known, much of the light output from the light-emissive layer in the OLED is absorbed within the device. Because light is emitted in all directions from the internal layers of the OLED, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
OLED devices generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer and the hole transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EL can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc. It has been found, however, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
Referring to FIG. 8, an OLED device as proposed in the prior art is illustrated having a substrate 10 (either reflective, transparent, or opaque), a reflective first electrode 12, one or more layers 14 of organic material, one of which is light-emitting, a transparent second electrode 16, a gap 18 and an encapsulating cover 20. The encapsulating cover 20 is transparent and may be coated directly over the transparent electrode 16 so that no gap 18 exists. It has been proposed to fill the gap with polymeric or desiccating material. Such polymers and desiccants typically will have indices of refraction greater than or equal to that of the substrate 10 or encapsulating cover 20, and it is generally proposed to employ materials having indices of refraction matched to that of the encapsulating cover to reduce interlayer reflections. Light emitted from one of the organic material layers 14 can be emitted directly out of the device, through the encapsulating cover 20, as illustrated with light ray 1. Light may also be emitted and internally guided in the encapsulating cover 20 and organic layers 14, as illustrated with light ray 2. Additionally, light may be emitted and internally guided in the layers 14 of organic material and transparent electrode 16, as illustrated with light ray 3. Light rays 4 emitted toward the reflective first electrode 12 are reflected by the reflective first electrode 12 toward the cover 20 and follow one of the light ray paths 1, 2, or 3. In some prior-art embodiments, the first electrode 12 may be opaque and/or light absorbing. In an alternative prior-art device, light is emitted through the substrate and light is trapped therein rather than in the cover. In this case, substrate 10 and electrode 12 are transparent and electrode 16 may be reflective.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. Such techniques include the use of diffraction gratings, brightness enhancement films having diffractive properties, reflective structures, and surface and volume diffusers. The use of micro-cavity techniques is also known. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device. Moreover, diffractive techniques cause a significant frequency dependence on the angle of emission so that the color of the light emitted from the device changes with the viewer's perspective. Scattering techniques are also known and described in, for example, US 2006/0186802 entitled “OLED device having improved light output” by Cok which is hereby incorporated in its entirety by reference.
It is also known to combine layers having color-conversion materials with scattering particles to enhance the performance of the color-conversion materials by increasing the likelihood that incident light will interact with the color-conversion materials, thereby reducing the concentration or thickness of the layer. Such combination may also prevent light emitted by the color-conversion material from being trapped in the color-conversion material layer. US20050275615 A1 entitled “Display device using vertical cavity laser arrays” describes such a layer as does US20040252933 entitled “Light Distribution Apparatus”. US20050012076 entitled “Fluorescent member, and illumination device and display device including the same” teaches the use of color-conversion materials as scattering particles. US20040212296 teaches the use of scattering particles in a color-conversion material layer to avoid trapping the frequency-converted light. Co-pending, commonly assigned U.S. Ser. No. 11/361,094, filed Feb. 24, 2006, entitled “Light-Scattering Color-Conversion Material Layer” by Cok which is hereby incorporated in its entirety by reference describes integral light-scattering color-conversion material layers.
US20050007000 A1 entitled “Brightness and contrast enhancement of direct view emissive displays” describes emissive displays including a plurality of independently operable light emitters that emit light through one or more transmissive layers. The emissive displays further include elements disposed between the light emitters and the transmissive layers to frustrate total internal reflections that can occur at one or more of the interfaces created by the transmissive layers, such as at an interface between the light emitter and a transmissive layer or at an interface between a transmissive layer and air. By frustrating total internal reflections, the brightness of the emissive display can be enhanced. Elements for frustrating total internal reflections include volume diffusers, surface diffusers, microstructures, and combinations of these or other suitable elements.
As taught in the prior art, classic scattering theory employs arrays of spheres. If such spheres are arranged adjacent to each other to form a layer, the ratio of the volume of the spheres (Vp=(4*π*r3)/3) divided by the volume of the layer (VL=(2*r)3) is equal to π/6 or 0.5236. U.S. Pat. No. 5,955,837 entitled “Electroluminescent illumination system with an active layer of a medium having light-scattering properties for flat-panel display devices” describes the use of a half-monolayer of scattering particles on the substrate of a bottom-emitting electro-luminescent device. The volume ratio of such a layer is at most one half of a classical mono-layer array of contacting spheres, or approximately 0.26. While optimizing the combination of extraction of light trapped in the substrate and non-scattering of non-trapped light for the disclosed devices, such a material layer may not scatter light trapped in the organic and electrode layers, and is difficult to form. Moreover, as the amount of trapped light relative to the amount of emitted light increases, the relative amount of desirable scattering increases so that the described half-monolayer of scattering particles may not optimally extract all of the emitted light from OLED devices of interest. It is also true that the organic electro-luminescent materials, reflective electrodes, and transparent electrodes all absorb some light. Hence, it can be desirable to scatter trapped high-angle light as soon as possible to minimize absorption. Additionally, experiments performed by applicant have shown that such volume ratios of a practical scattering layer does not extract light optimally for some OLED device structures.
There is a need therefore for an improved organic light-emitting diode device structure and scattering layer that further improves the efficiency of the device.